1. Field of the Invention
The invention relates generally to electronic devices, and more particularly to systems and methods for improving the performance of switching amplifiers by adding an ultrasonic signal to an input audio signal before amplifying the audio signal.
2. Related Art
Pulse-width modulation (PWM) amplifiers are sometimes used to amplify audio signals. In a two-level PWM modulation scheme, a pair of signals (A, B) are used to switch a pair of FETs on and off to thereby pull the voltage across the load (speaker) alternately up or down. (See FIG. 1.) The signals A and B each consist of a series of pulses of varying widths. If the audio signal is increasing, the high-side signal (A) will have wider pulses and the low-side signal (B) will have narrower pulses. Similarly, if the audio signal is decreasing, the high-side signal will have narrower pulses and the low-side signal will have wider pulses.
Some PWM amplifiers use a three-level (class BD) modulation scheme instead of a two-level scheme. In a three-level PWM modulation scheme, the pair of signals A and B are used to switch two pair of FETs on and off. Rather than only being able to pull the speaker voltage up or down, the FETs may also be controlled to couple both sides of the load to the same voltage (i.e., to not pull the voltage up or down.) An exemplary system is shown in FIG. 2.
A three-level modulation scheme can have two sources of distortion, including coincidence distortion and dead-time distortion. Coincidence distortion is caused because, at low signal levels, the two sides of the bridge switch almost simultaneously. Dead-time distortion shows up, or is most noticeable, at low signal levels because the dead-times occur while the output current is close to 0. Dead-time distortion normally causes the signal to be flattened at the zero-crossings.